Especially biological products which can only be produced in small quantities have to be temporarily stored before further processing. This is usually carried out by means of freezing in order to maintain the quality of the product during storage. The controlled thawing of the products serves not least to even out the product properties which, particularly in the case of biological products, can vary greatly from batch to batch. To achieve a small range of fluctuation of the product properties, it is frequently necessary for several hundred to a few thousand batches to be thawed together and mixed with one another.
This is frequently carried out in vertical stirred vessels with jacket heating, because in this way no seal is touched by the product when the stirrer is operated from above. The ice pieces are taken from the storage containers and introduced into the stirred vessel. Breaking up the ice pieces is often not advisable for hygienic reasons. It is frequently also the case that no additives are required or are even permitted, so that the melting process in the stirred vessel takes place initially without a liquid phase. The ice pieces are set into rotary motion by the stirrer and slide along the heated wall of the vessel. The molten material collects the bottom of the vessel. Owing to the size of the ice pieces relative to the internal diameter of the stirred vessel (the ratio of edge length to diameter can even exceed 1/10), the installation of baffles is not advisable. There is therefore a risk of the ice pieces jamming and leading to mechanical failure of the stirrer.
For this reason and to intensify heat transfer via the wall, stirrers which travel around the wall, for example helical stirrers, are usually employed. Owing to the low friction between water ice and the wall of the vessel due to formation of a water film and the movement of the stirrer used around the wall, the ice pieces are not mixed with one another. In extreme cases, it is even possible for ice pieces which have previously been partially thawed on the external surfaces but are still deep frozen on the inside to freeze together and form a coherent mass which rotates together with the stirrer without any relative motion. The ice pieces which touch the wall melt at the wall until an air gap having a width of a few millimetres has been formed. In this mode of operation, there is hardly any formation of fresh contact areas, so that the thawing process takes longer. During this process, the thawed material is subjected to a high thermal stress for a prolonged time, which can have an undesirable effect on the product properties.
In all cases, the ice pieces float on the surface because of their lower density compared to the molten material. The previously mentioned low wall friction leads to the impetus necessary for axial transport not being able to be introduced. A division into a molten fraction in the lower region and still solid ice pieces rotating together with the stirrer in the upper region is the result. Heat exchange between these two regions is hindered by the lack of axial transport of the ice pieces, which has the consequence that the molten product is subjected to a high thermal stress over a long period of time.
It was an object of the invention to accelerate the melting process of mechanically and thermally unstable substances in aqueous solutions and to avoid mechanical and thermal stress as far as possible. Particular attention was to be paid to ensuring that local overheating due to separation of solid and liquid phases is nevertheless avoided at a low specific power input as high space-time yield by means of low stirrer speeds.
This object is achieved by a method which prevents separation of the two phases by use of a horizontal mixer in which the stirrer elements do not rotate about a vertical axis but make vertical mixing possible.